St70 clone problems

[dodeca12]

Tom, I double checked and yes, they are exactly 15.1 ohm. Also double checked the 6 to 5 pin resisters I put in and all are 1000 ohms.
Although I had the output tubes checked + they tested good I'm wondering if one of them is the problem. Could the tester have missed something.
Thanks for the advice Terry. Would I disconnect the connection between the 1/8 pins of the outputs and add another pot to accomplish this?

[TerrySmith]

No, you don't need any more pots. Instead of one 15.1 ohm resistor for both tubes, you will end up with one 10 ohm resistor for each tube. Just connect pins 1&8 together with one end of the resistor and a piece of wire. Connect the other end of the resistor to ground. Connect the other end of the wire to an unused pin on the test socket. Then repeat for the other three tubes.

Look in the Docs & Mods section, and there in the assembly manual, page 4, is a schematic of this mod.

This way when you set the bias, you would see .5v on each tube if everything is normal. If you saw .3v and .6v for one channel, you could have a mis-matched pair or one weak tube.

[dodeca12]

Finally got my st70 running a week ago last saturday. While waiting for my rectifier tube and 7199s from triode I decided to clean up the wiring. Since there were about 6 types of wire (cloth,vinyl, stranded,solid ) went ahead and rewired almost everything to and from the driver board with 20g teflon wire.
Found another problem, the heater wire to v6, pin 7, was never soldered. It was making pressure contact that became intermitant as I was going thru the amp.
A small intuitive voice told me to check the cap board just before I was going to throw in the tubes. Made a few checks of the caps and resisters, then as I was proceeding to lock it down I noticed another unsoldered wire that went to the bias pot.
Put all the tubes in, turned it on, and got five feet away and heard a wah-wah-wah-hmmmmmm. Turned up the volume and the left channel sounded fine, the left like a weak transister radio with hum. After an initial what did i do wrong, reviewed evrything I had done and tried switching tubes. Hum switched channels. Tried another pair of matched 7199s, this time both channels sounded distorted w/hum.
Last resort installed the 7199s (one of which tested with a short) that came with the amp. Flipped it on and sounded good. After 6 hours the amp really souned good. Seven hours and couldn't suppress a smile. Loaded an hdcd and the smile got bigger.
Thanks Everyone for the help. Terry, I installed the bias circuit you suggested, love it. First biasing there was a 7/100s difference, after a few swaps one side had a 3/1000s diff., and the other 2/1000s. First time I've ever done this but tend to think thats a fairly good match.
Got my Pas done tonight but will wait to tomorrow to plug and play. Sure don't trust the sovtek 7199s I got from triode. Three of four bad. Might consider modifying the driver board sooner than planned, and returning these 7199s for tubes that match the board I decide on. Richard

[Shannon Parks]

Flipped it on and sounded good. After 6 hours the amp really souned good. Seven hours and couldn't suppress a smile. Loaded an hdcd and the smile got bigger.

Good job sticking with it, Richard!

Shannon